ABSTRACT The retina converts light into an electrical signal through a series of biochemical steps collectively referred to as phototransduction. This signal is eventually relayed to the visual cortex of the brain, where visual perception occurs. Photoreceptor cells are able to respond to light throughout our lives because they have the ability to regenerate proteins as well as a light-sensitive chromophore. The long-term objective of our research is to elucidate the molecular reactions involved in phototransduction, including those directly involved in the regenerative capability of photoreceptor cells. Phototransduction serves as a prototype for a multitude of G protein-mediated signal transduction events initiated by activation of G protein-coupled receptors (GPCRs) and thus understanding of this process is broadly applicable to other signal transduction cascades. Our continuing investigation of phototransduction will focus on selected proteins in this pathway. Mutations in the genes encoding these particular proteins are among the main causes of blinding diseases in humans. We propose three thematically linked specific aims in this application: (1) Determine the structures of phototransduction-specific proteins and their regulation by posttranslational modifications. Understanding the 3-dimensional architecture of transduction proteins and their posttranslational modifications will yield an unparalleled wealth of atomic detail related to their function. (2) Elucidate the structural/biophysical changes involved in rhodopsin activation. Studies proposed herein will provide new biochemical and physiological insights into critical events required for receptor activation and contribute to our understanding of GPCR signaling. (3) Use pathogenic mutations of rhodopsin to determine precisely how changes in its structure affect the viability of rod photoreceptor cells. By elucidating the molecular details of abnormal cellular processing of mutated rhodopsin, we will better understand the ensuing pathology as well as develop a rational approach to alleviate retinal dystrophies related to mutations in the opsin gene.